An active matrix organic light emitting diode (AMOLED) type display due to its higher refresh rate and its shorter response time is widely used in display apparatus. Organic light emitting diode elements are configured to emit light beams in the AMOLED type display. The AMOLED includes a plurality of pixel units and a plurality of pixel driving circuits, which correspond to the pixel units respectively. The pixel driving circuit is configured to drive the brightness of a corresponding one of the pixel units, and a driving control circuit is configured to detect the pixel driving circuits. Referring to FIG. 6, a typical pixel driving circuit and its driving control circuit for controlling the pixel driving circuit of an AMOLED display is shown. The pixel driving circuit 110k includes a switching transistor MN1, a driving transistor MN2, and a storage capacitor C1. The switching transistor MN1 receives a scan signal from a corresponding scan line SELi, and turns on for loading a data signal on a corresponding data line Dk when the scan signal is in an active state, such as a high level voltage. The storage capacitor C1 is being charged by the loaded data signal. When the switching transistor MN1 turns off, the storage capacitor C1 discharges and the driving transistor MN2 turns on for providing a current to the OLED, thus the OLED emits light. However, driving transistors in the pixels of the OLED display may be subject to manufacturing variations or operating variations. Due to such variations, transistor threshold voltages between different display pixels may vary. Variations in transistor threshold voltages can cause the pixels to produce amounts of light that do not match a desired image. A method for compensating the transistor threshold voltage can solve the above-mentioned light variation problem. In this method, a detecting time period is provided for detecting parameters in the pixel driving circuit, such as a threshold voltage of the driving transistor MN2, and the current provided to the OLED before a displaying period. During the detecting time period, the switching transistor MN1 turns on and loads different testing voltages for detecting the threshold voltage of the driving transistor MN2 and the current provided to the OLED, and a time of a potential of the node VSO increased to the predetermined voltage, which is connected between a gate electrode of the driving transistor MN2 and the OLED, is too long, thus a time of the detecting time period to be operated in a steady state for detecting the threshold voltage is too long.